Quantum cascade lasers (QCLs) are important semiconductor lasers, operating at room temperature, in the mid-infrared spectral range, from 3.5 to 20 microns, in terms of output power and wallplug efficiency. Wallplug efficiencies in excess of 20% have been demonstrated in the past few years, and devices producing multi-watt continuous-wave output power are now commercially available. As with any other kind of high-power edge-emitting semiconductor lasers, including diode lasers, one of the factors limiting the output power and reliability of QCLs is optical damage of laser facets, which has, for a long time, been the main roadblock, limiting the output power of commercially-available fully-packaged continuous-wave room-temperature QCLs to about 2 W for a reliability of >3000 hours.
The active region of a mid-infrared quantum cascade laser, which also constitutes the core of its dielectric waveguide, is typically about 5-15 μm wide and about 1-2 μm thick. The optical mode is therefore confined to an area, A, on the order of 10−6-10−7 cm2. The optical power density, or optical intensity, I=P/A, where P is the power travelling in the waveguide, increases as a function of the output power Pout. Because of the very small mode size, the optical intensity reaches very large values for high power QCLs. The output, or front, facet is more susceptible to optical damage than any other part of the device because, in the standard configuration with a high-reflectivity (HR) coated back facet and an anti-reflective (AR) coated front facet, the front facet is where the optical intensity is the highest. As an example, for a buried-heterostructure QCL with an active region width of 10 μm emitting 3 W of output power at a wavelength of 4.6 μm, the optical power density at the front facet is higher than 10 MW/cm2. In addition, the front facet is typically coated with a dielectric AR coating, which has a relatively low thermal conductivity, unlike the back facet whose HR coating typically contains a metallic layer, which enhances its thermal conductivity. However, optically-induced damage to the rear (HR-coated) facet is also possible.
The maximum attainable optical power before facet damage, Pmax, can be increased in at least two ways. One can improve the optical damage threshold power density by improved AR and/or HR coating design and/or deposition process to minimize optical absorption in the coating layers and/or enhance heat removal, or one can reduce the optical power density by increasing the output facet area. The most straightforward way to reduce optical intensity, I, at the facets at constant output power level is to utilize wider longitudinally uniform, or straight, devices. However, this approach has two major drawbacks. First, wider devices support a larger number of transverse optical modes and, therefore, have lower beam quality. Second, wider devices generate more heat per unit length, which results in a higher active region temperature and, hence, lower performance in high-duty-cycle and continuous-wave operation.